1. Field of the Invention
The present invention relates to an optical semiconductor device, and a manufacturing method thereof.
2. Description of the Related Art
For example, an optical fiber is connected to a laser module having a semiconductor laser device. In this case, it is important to efficiently couple laser light emitted from the laser module to the optical fiber. With regard to an emission surface of a semiconductor laser device, in the case where optical confinement in an optical waveguide is strong, the far field pattern (hereinafter, referred to as FFP) of the laser light is widened, and a coupling efficiency with respect to the optical fiber is decreased. Further, in a case where the optical confinement in the optical waveguide is strong in one of a longitudinal direction and a transverse direction, and is weak in the other thereof, the FFP accordingly has an elliptical shape which is wide in one direction and is narrow in the other direction, and thus, the coupling efficiency with respect to the optical fiber which has a circular cross-sectional shape is decreased. Thus, it is desirable to adjust the FFP to have a shape which is moderately narrow and is close to a circle by moderately alleviating the optical confinement in the optical waveguide and by allowing the optical confinement to have similar strengths in the longitudinal and transverse directions.
In order to alleviate the optical confinement in the optical waveguide and adjust the FFP of the semiconductor laser device, a spot-size converter is used. For example, “Electronics Letters, vol. 30, No. 11, pp 857-859, 1994” and “Journal of Selected Topics in Quantum Electronics, vol. 3, No. 6, pp 1308-1320, 1997” disclose a semiconductor laser device which includes a mesa width tapered spot-size converter. FIG. 12 is a diagram schematically illustrating a structure of a mesa width tapered spot-size converter in the related art. The rear surface in the figure corresponds to an incident surface of laser light, and the front surface therein corresponds to an emission surface of the laser light. In a case where a semiconductor laser device has a buried hetero-structure (hereinafter, referred to as a BH structure), the spot-size converter is disposed on the emission side of the semiconductor laser device, and the spot-size converter has a tapered structure in which the mesa width of a mesa stripe structure included in a core layer 112, decreases along an emission direction of the laser light. Due to the BH structure in which both sides of the mesa stripe structure are buried in a buried layer, the periphery of the core layer 112 having such a shape is surrounded by a clad layer 111. Since the light confined in the core layer 112 is leaked to the peripheral clad layer 111 as the mesa width is decreased, a mode field of the light propagating in a waveguide may be widened in a longitudinal direction and also in a transverse direction, and the FFP may be narrowed in a longitudinal direction and also in a transverse direction.
Further, “Journal of Selected Topics in Quantum Electronics, vol. 3, No. 6, pp 1308-1320, 1997” discloses a semiconductor laser device which includes a thick film tapered spot-size converter. FIG. 13 is a diagram schematically illustrating a structure of a thick film tapered spot-size converter in the related art. In a similar way to FIG. 12, the front surface in the figure corresponds to an emission surface of laser light, and the periphery of a core layer 112 is surrounded by a clad layer 111. The spot-size converter shown in FIG. 13 has a tapered structure in which the thickness of the core layer 112 is gradually decreased along an emission direction. Since the light confined in the core layer 112 is leaked to the peripheral clad layer 111 as the layer thickness is decreased (the layer becomes narrow), a mode field of the light may be widened and the FFP may be narrowed, in a similar way to the spot-size converter shown in FIG. 12.
Further, for example, JP09-102651 A and “Journal of Selected Topics in Quantum Electronics, vol. 3, No. 6, pp 1308-1320, 1997” disclose a dual mesa tapered spot-size converter which includes a two-layered core layer having an upper core layer and a lower core layer. The refractive index of the upper core layer is higher than the refractive index of the lower core layer, and the mesa width of the upper core layer is decreased along an emission direction of laser light, in a similar way to the core layer 112 shown in FIG. 12. On the other hand, the mesa width of the lower core layer is increased along the emission direction of the laser light. On the incident side of the spot-size converter, the confinement of light is strong in the upper core layer of which the refractive index is high, and the light transits from the upper core layer to the lower core layer as the mesa width of the upper core layer is decreased. Since the refractive index of the lower core layer is lower than the refractive index of the upper core layer, the confinement of the light transited to the lower core layer becomes weak, and on the emission side of the spot-size converter, the mode field of the light may be widened and the FFP may be narrowed.